Packet communications have evolved to a point where voice sessions, or calls, can be supported with essentially the same quality of service as that provided by circuit-switched communications. Packet communications are generally supported over packet subsystems, which were initially supported by local area networks, but are now supported by wireless local area networks (WLANs). Using WLAN access, user elements can support voice sessions using packet communications while moving throughout the WLAN. As such, WLAN access provides users the same freedom of movement within a WLAN as cellular access provides users within a cellular environment.
In many instances, the coverage areas provided by WLANs and cellular networks are complementary. For example, a WLAN may be established within a building complex in which cellular coverage is limited. Given the localized nature of WLAN coverage, cellular networks could bridge the coverage gaps between WLANs. Unfortunately, WLAN access technology is independent of cellular access technology. Cellular networks generally support circuit-switched communications, and WLANs support packet communications. As such, user elements have been developed to support both cellular and WLAN communications using different communication interfaces. With these user elements, users can establish calls via the cellular network and WLAN using the respective communication interfaces; however, calls established via the cellular network are not easily transferred to the WLAN, and vice versa.
During a transfer of a call from the cellular network to the WLAN, it is preferable to establish a new connection via the WLAN interface of the user element prior to dropping the old connection via the cellular network, and vice versa. Unfortunately, there are a few scenarios where maintaining the old connection in the old subsystem before establishing the new connection in the new subsystem is difficult, if not impossible. For example, the user element may not be able to support cellular and WLAN communications at the same time. Even if the user element is capable of supporting simultaneous cellular and WLAN communications, there may be breaks in coverage during a call as the user element moves from a cellular coverage area to a WLAN coverage area.
Even though breaks may be unavoidable during transfers from one subsystem to another, there would be benefit in reestablishing the voice path for the call via the new subsystem without dropping the call. Although the user may experience a break in communications, there would be no need to initiate a new call. As such, the inconvenience to the user is minimized.
Accordingly, there is a need for a technique to effectively support calls for a user element over both cellular networks and WLANs as well as provide transfers for established calls between the respective networks. There is a further need for a technique to maintain the call, even if a brief break in the actual voice path is required during the transfer due to the configuration of the user element or a break in coverage. The need for such a technique is particularly felt for transfers between packet-based third generation subsystems and circuit-switched subsystems.